Communication devices such as User Equipment (UE) are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
UEs may further be referred to as wireless terminals, mobile terminals and/or mobile stations, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is being served by a network node. A cell is the geographical area where radio coverage is provided by the network node.
The network node may further control several transmission points, e.g. having Radio Units (RRUs). A cell may thus comprise one or more network nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. A network node is an entity that controls one or more transmission points. The network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each network node may support one or several communication technologies. The network nodes communicate over the air interface operating on radio frequencies with the UEs within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.
Long Term Evolution (LTE) is a radio access technology standardized by the 3rd Generation Partnership Project (3GPP). In LTE, network nodes, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks. In LTE the cellular communication network is also referred to as E-UTRAN. The standard is based on Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) in the uplink.
An E-UTRAN cell is defined by certain signals which are broadcasted from the network node. These signals contain information about the cell which can be used by UEs in order to connect to the network through the cell. The signals comprise reference and synchronization signals which the UE uses to find frame timing and physical cell identification as well as system information which comprises parameters relevant for the whole cell.
A UE trying to connect to the wireless network must thus first detect a suitable cell, as defined in 3GPP TS 36.304 v11.5.0. The UE may be in either idle state, which is also referred to as IDLE or RRC_IDLE, or in connected state, which state is also referred to as CONNECTED or RRC_CONNECTED. When the UE is in RRC_IDLE, it monitors a paging channel, which paging channel is part of a Paging Control Channel (PCCH) at a logical level, a Paging Channel (PCH) on a transport channel level and a Physical Downlink Shared Channel (PDSCH) on a physical channel level. While doing so it typically also performs a number of radio measurements which it uses to evaluate the best cell, such as Reference Signal Receive Power (RSRP), Reference Symbol Received Quality (RSRQ) or Received Signal Strength Indicator (RSSI). This is performed by measuring on received reference signals and/or parts of a spectrum which comprises reference signals sent by cells. This may also be referred to as “listening” for a suitable cell.
A suitable cell is commonly a cell which has RSRQ or RSRP above a certain level. The cell with the highest RSRP or RSRQ may be referred to as the best cell or the best suitable cell. Listening for a suitable cell may comprise searching for reference signals transmitted from the network node in an OFDM subframe. When the best suitable cell is found the UE performs random access, according to a system information for the cell. This is done in order to transmit a Radio Resource Control (RRC) connection setup request to the network node. Assuming the random access procedure succeeds and the network node receives the request, the network node will either answer with an RRC connection setup message, which acknowledges the UEs request and tells it to move into RRC connected state, or an RRC connection reject, which tells the UE that it may not connect to the cell. In RRC connected state the parameters necessary for communication between the network node and the UE are known to both entities and a data transfer between the two entities is enabled.
In LTE all packets are delivered using the IP protocol. This means that also traditionally circuit switched services such as conversational voice will make use of fast scheduling and will be served using Voice over IP (VoIP). In a typical VoIP arrangement a voice encoder on the transmitter side encodes the speech into voice packets, each with the typical speech duration of 20 ms, as mandated by GSMA IR.92. VoIP may for LTE networks also be referred to as Voice over LTE (VoLTE). VoLTE enables LTE networks to provide voice services.
The mouth-to-ear delay introduced by the transport scheduling and transmission of the VoLTE packets is one of the main factors when determining the experienced speech quality. This causes a relatively tight delay budget for VoLTE to ensure good speech quality. The normal delay budget in the eNB is 80 ms. Up to the limit of the delay budget the speech quality is good, which means that in general it is sufficient to schedule a VoLTE user once every 40 ms hence bundling two packets. Such a scheduling method allows for a good balance between efficient usages of resources, good speech quality and enhanced battery performance. The VoLTE service uses a Real-time Transport Protocol (RTP) or a Secure Real-time Transport Protocol (SRTP) to transport voice data between the clients. Hereinafter, if not else mentioned the embodiments discussed for RTP shall be equally applicable to SRTP.
RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of-service for real-time services. The data transport is augmented by a Real-time Transport Control Protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality. RTP and RTCP are designed to be independent of the underlying transport and network layers. The protocol supports the use of RTP-level translators and mixers.
The VoLTE clients monitor the RTP packet flow in order to detect gaps. If the RTP gap is long enough, usually 10 seconds, the VoLTE client will initiate a call release and set a release cause to “RTP Timeout”. Thereby the VoLTE call will be dropped.
In order to re-establish the RRC connection after a failure in operation, the UE may perform a RRC Connection Reestablishment procedure, which involves the resumption of operation on a Signalling Radio Bearer 1 (SRB1), which is used to transfer RRC messages in the Downlink Control CHannel (DCCH), and the re-activation of security.
A UE in RRC_CONNECTED mode, for which Access Stratum (AS) security has been activated, may initiate the RRC Connection Reestablishment procedure in order to continue the RRC connection. The connection re-establishment succeeds only if the concerned cell is prepared i.e. has a valid UE context. In case E-UTRAN accepts the re-establishment, SRB1 operation resumes while the operation of other radio bearers remains suspended. When AS security has not been activated, the UE moves directly to RRC_IDLE instead of initiating the procedure. The purpose of AS security is to securely deliver RRC messages between a UE and a network node in the control plane and IP packets in the user plane using AS security keys. The AS security keys are derived from a network node key and new keys are generated every time a new radio link is established, such as when RRC state moves from idle to connected. After the AS security setup is completed, the UE and the network node get to share an RRC integrity key, an RRC encryption key and a user plane encryption key. Encryption and integrity protection using these keys are performed at a Packet Data Convergence Protocol (PDCP) layer. The RRC integrity key and the RRC encryption key are used to securely deliver RRC messages in the control plane through a Signaling Radio Bearer (SRB) over radio links. The RRC messages are integrity protected using the RRC integrity key and encrypted using the RRC encryption key at the PDCP layer before being sent. The user plane encryption key is used to securely deliver IP packets in the user plane through a Data Radio Bearer (DRB) over radio links. The IP packets are encrypted using the user plane encryption key at the PDCP layer before being sent.
The PDCP layer exists in the UE and network node and is a part of LTE air interface control and user planes. The PDCP sublayer provides several services and functions both on the control plane and on the user plane. The main services and functions of the PDCP sublayer for the user plane include:                Header compression and decompression: This only applies for RObust Header Compression (ROHC);        Transfer of user data;        In-sequence delivery of upper layer Protocol Data Units (PDUs) at PDCP re-establishment procedure for Radio Link Control Acknowledge Mode (RLC AM);        Duplicate detection of lower layer Service Data Units (SDUs) at PDCP re-establishment procedure for RLC AM;        Retransmission of PDCP SDUs at handover for RLC AM;        Ciphering and deciphering;        Timer-based SDU discard in uplink.The main services and functions of the PDCP for the control plane include:        Ciphering        Integrity Protection;        Transfer of control plane data.        
One of the biggest problems with VoLTE in live networks today is call drop. Call drop for VoLTE is always affecting the end-user. There is a big difference having a drop in a mobile broadband UE. The Mobile Broadband UE is often active even though the end-user might not have initiated the activity. Examples of such activities may be push mail, facebook updates, etc.
However, this is not the case for VoLTE. As soon as a VoLTE call has been initiated there is always an end-user expecting to get a good voice service, without interruptions.
A VoLTE UE may drop due to many reasons; however one of the most common is an RTP-Timeout, where the uplink or downlink RTP flow has stopped for some reason, creating an RTP gap.
The reasons for the RTP gaps may be many and might also be caused by the EPC core network, IMS, transport or RAN.
Usually the RTP gaps are caused by poor conditions in the RAN. The weakest link in the communication with the UE is the UL. In UL the UEs output power is usually the limiting factor, giving that very often it is the UL failing during a UE-eNB communication. The failed communication might be caused by high UL interference, high path loss, high load or other radio related reasons, like fading, etc.
The VoLTE service is using RLC Unacknowledge Mode (UM) to transfer the voice and silence data, which does not have delivery guaranteed as RLC Acknowledge mode (AM) has. In this scenario the RTP client in the receiving end, such as e.g. the UE, will detect the RTP gap in the DL and normally after about 10 seconds (depending on configuration) it will initiate a release of the call with the release cause “RTP-Timeout” and the call will be dropped.